This invention relates generally to an electrophotographic printing machine, and more particularly concerns a method and tool for measuring the gap between the trim bar and magnetic roll.
In an electrophotographic printing machine, a photoconductive member is charged to a substantially uniformed potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. This forms a powder image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the marking particles thereto in image configuration.
In the foregoing type of printing machine, a development system is employed to deposit developer material onto the electrostatic latent image recorded on the photoconductive surface. Generally, the developer material has toner particles adhering triboelectrically to coarser carrier granules. Typically, the toner particles are made from a thermoplastic material while the carrier granules are made from a ferromagnetic material. Alternatively, a single component magnetic material may be employed. A continuous supply of toner particles must be available to be capable of copying a large number of original documents or producing multiple copies of the same original document. This is necessary in order to ensure that the machine is not shut down at relatively short intervals due to the lack of toner particles. This is achieved by storing a supply of toner particles in a toner container and dispensing additional toner particles into one end of the developer housing chamber. The toner particles are then transported across the chamber of the developer housing and advanced to a developer roller. The developer roller transports the toner particles closely adjacent to the photoconductive member and the latent image attracts toner particles thereto.
In magnetic roll developing, a hollow tube or roll containing fixed magnets is used to move the developer to the photoreceptor. As the tube rotates around the magnets and through a sump of developer, the magnets attract the metallic carrier beads in the developer. The carrier beads closest to the magnetic roll become magnetized, enabling more carrier beads to stick. As a result, the developer builds up to form bristles like a brush. This attraction of developer to the magnetic roll is called a magnetic brush. The magnetic force on the magnetic roll is just enough to allow the developer to build up on the roll. The magnetic force has little effect on the charge either on the toner or on the photoreceptor.
As the magnetic roll or tube continues to rotate, it moves the developer brush past a trim bar. The trim bar limits the length of the brush by knocking off the portions of the developer bristles that extend beyond a certain length. The trim bar ensures that just the tip of the brush touches the photoreceptor.
The gap between the trim bar and the magnetic roll must be exactly determined so that neither too much nor too little developer material is brought in contact with the photoreceptor surface. If the exact amount of developer material is not precisely metered by the trim bar, the print quality of the final printed product can suffer.
U.S. Pat. No. 4,823,071 to Ding et al. discloses a capacitive measuring system for measuring the distance between two relatively moving parts based on the measurement of capacitive charge, comprising a measuring sensor which forms one plate of the capacitor whose gap is to be measured. The position of the sensor face in the fixed part of the casing is known, thus the plate distance of the gap provides a direct measurement of the gap.
U.S. Pat. No. 4,311,959 to Riessland et al. discloses a capacitive probe with a special shielding which allows measurement of small differences in distance which correspond to small differences in capacitance. A first and second electrode are located within the shield which protects a coupling pin from the shield. The shield has a decoupling projection between the first and second electrodes so that the capacitance between the electrodes and between the shield and coupling pin are very small. With the capacitive measuring probe, distances to a conductive material are measured, as well as the position of the bordering edges of the conductor material.
U.S. Pat. No. 4,063,167 to Duly discloses an apparatus for the non-contact measurement of the clearance between the blade tips of a rotating turbine and the surrounding housing, by the use of a capacitance probe. The probe consists of two metallic conductors insulated from one another and arranged to form the plates of the capacitor. Passing a blade tip in close proximity to the two plates of the probe changes the capacitance.
U.S. Pat. No. 3,030,5788 to Lessman discloses a device for measuring elements with accuracy of less than one millionth of an inch. The device comprises a) a carriage movable by a lead screw, b) a pair of helical capacitor plates carried by the lead screw, and c) a measuring screw driven by the lead screw and having a thread between the capacitor plates to form capacitors therewith, which may be balanced relative to one another to precisely ascertain the position of the measuring screw relative to the lead screw.
U.S. Pat. No. 4,935,700 to Garbini et al. discloses a fringe field capacitive distance sensor for measuring the distances between a probe and the surface of a conductive body over a wide range. A sensor encased in a ceramic material is comprised of a conductive plate element mounted by the probe, the thickness of the plate element being substantially less than the height and width of the plate element. The plate element is positioned adjacent to the body being measured and the fringe field capacitance between the plate and body is measured.
U.S. Pat. No. 3,048,775 to Calvert discloses an apparatus for measuring small distances and/or measuring the amplitude of mechanical vibrations. The apparatus for measuring the spacing between a pair of plates of a capacitor comprises a) a high gain amplifier arranged so that the capacitor may be connected as a feed back circuit across the amplifier to feed a current into the amplifier input dependent on the amplifier output voltage, b) a source of alternating current of constant amplitude coupled to the amplifier input in opposition to the feed back current, and c) indicating means coupled to the amplifier output.
While the above-mentioned measuring systems allow for measuring distances such as with a capacitor type sensor, there continues to be a need for a system for measuring the gap between a magnetic roller and an adjacent member, such as a trim bar.